<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>
<body>
    <script>
        function card() {
           let shuru=Number(prompt('输入银行卡余额'))
           let eat=Number(prompt('输入银行卡余额'))
           let live=Number(prompt('输入银行卡余额'))
           let money=shuru-eat-live
           return money
        }
       document.write(card());
    </script>
</body>
</html>